The superheated steam occurring in chemical-processing and power plants must be cooled when it is employed to drive turbines or when it is usually necessary but no longer desired in special operating conditions when saturated steam is used at the same or a lower pressure.
The coolers employed are either spray-type coolers, which operate with degassed water or pure condensed steam, or surface coolers, wherein the superheated steam is cooled indirectly by evaporating water at an equal or lower pressure (Dubbel Taschenbuch fur den Maschinenbau, 14th ed., 1981, p. 606). The use of spray-type coolers is limited in some situations when not enough satisfactory water is available. Surface coolers have advantages over spray-type coolers in that the quality of the steam being cooled remains constant and that steam is simultaneously generated by the cooling process. Separating the superheated steam being cooled into component currents facilitates regulating the temperature and volume of the resulting saturated steam. Surface coolers are mainly nested-tube heat exchangers (op. cit., p. 545).